Display units provided with cathode-ray tubes are increasingly being used in office work, in computer settings, process control, in laboratories etc. Therefore, special attention has begun to be paid to the detrimental electric and magnetic fields caused by visual display units and monitors especially in continuous and long-term use. The official recommendations will most likely determine limit values for both the magnetic fields and the electric fields which are in the low-frequency zone about 0 to 2 kHz (band 1) and in the high-frequency zone about 2 to 400 kHz (band 2).
The cathode-ray tube produces the band 2 type of an electrical field generally in three different ways. First, the horizontal deflecting windings of the deflection coils of the tube and the lead coupled thereto create around them an electric field proportional to the voltage in them. This voltage is known to be high, that is, a pulse-like voltage of about 1000 volts.
Secondly, the capacitance between the deflection coil and the anode of the tube provides the anode with a voltage pulse proportional to the voltage in the coils of the deflection coil, to the capacitance between the anode and the deflection coil, and to the capacitance between the anode and the ground. Thus a capasitive voltage division is created as the result of which the above-mentioned voltage pulse provides the anode with a voltage pulse according to band 2 and thus an electric field around the anode.
Thirdly, while the tube is functioning, an electron beam discharges the capacitance between the anode and the ground, which is recharged each time during a return pulse. Thus a line rate voltage resembling a saw-tooth wave is created for the anode. The voltage in turn creates an electric field of the type according to band 2.
The electric field created by the deflection coil and the leads coupled thereto can be eliminated in a relatively easy manner by packaging the cathode-ray tube in an electrically conductive housing of metal or coated plastic. Thus the electric field forward of the anode remains the only problem. This field can be attenuated by installing a conductive, light-permeable panel in front of the screen and by grounding it. This method is a very expensive solution, however. Another alternative is to increase the capacitance between the anode and the ground by a separate, high-capacitance high-voltage capacitor. This solution is also expensive and makes the high-voltage transformer impractically bulky.